0605 GET_WEAPONTYPE_FOR_MODEL should allow negative numbers

[x87]

For consistency with 032B, 0605 should also allow for negative numbers to be given as an argument. In case of 032B the game resolves the model index using the CTheScripts::UsedObjectArray, where -1 is the first model on the list, -2 is the second, etc.

This is a purely cosmetic change.